Method and apparatus for covering a pressure vessel

ABSTRACT

An apparatus disclosed herein relates to a pressure vessel cover comprising, a periphery of the cover, a first surface of the periphery for sealing on a first side of the cover and a second surface on a second side of the cover opposite from the first side, a third surface on the first side of the cover and located radially inwardly from the periphery, the third surface being displaced axially from the first surface in the direction of the second surface, a fourth surface on the second side of the cover radially inwardly positioned from the periphery, and a bore through the cover in fluidic communication with the third surface and the fourth surface.

BACKGROUND OF THE INVENTION

A chamber that contains a fluid within the chamber at higher pressures than is on an exterior of the chamber is often referred to as a “pressure vessel.” Pressure vessels may be used to house valves, which control the flow of the fluid through the pressure vessel. Such fluid flow may be for the transport of steam, for example, in the case of a steam turbine power plant. Housing the valves within the pressure vessel makes necessary a cover through which access to the valves is possible for purposes of servicing the valves, for example. The cover must form and maintain a seal to the pressure vessel under the various operating conditions that it will be subjected to, such as, elevated pressures and temperatures as well as cycles of varying pressures and temperatures.

Variable flow “control valves” and only fully-opened or fully-closed “stop valves” are commonly used in steam turbine power plants. With the use of these valves it is desirable to have a bore through the cover to allow for actuation of the valves, through the bore, from outside of the pressure vessel. Locating the actuator external to the pressure vessel, improves accessibility to the actuator, simplifying servicing of the actuator since the seal, of the cover to the pressure vessel, need not be disturbed.

Covers that incorporate bores tend to be very thick heavy plates of metal. Thick covers are used to minimize deformations of the cover resulting from stresses from pressure loading and from thermal bending induced from temperature variations across the thickness of the cover. Large deformations can cause failures of the bolts, which fastened the cover to the vessel, or the cover itself, due to low cycle fatigue (LCF), and therefore need to be minimized.

Pressure vessel covers with bores have typically been flat to accommodate and align the valve components that abut surfaces of the cover. Such components may include an actuator yoke, on the external surface of the cover, and a balance chamber and bushing holder, on the interior surface of the cover for example. These valve components abut flat surfaces surrounding the bore to provide alignment of the actuator externally and the valve head to valve seat internally.

Pressure vessels with thick flat covers with bores, however, continue to have failures due to stresses. Such failures include LCF bolt failures that result from the cyclic application of bending stresses. Some bending stresses occur during turbine start up when hot steam first reaches the interior surface of the cover. During this transient condition, the interior surface of the cover expands with the increase in temperature before the rest of the cover does. This expansion causes the cover to dish inward toward the pressure vessel. Such dishing causes a peripheral portion of the cover, through which the bolts pass, to also dish resulting in a bending stress being applied to the bolts.

Conversely, the cover is forced to dish outwardly after the cover and bolts reach a steady state operating temperature and full steam pressure is applied to the interior surface. Such dishing, in the opposite direction than that from the turbine start up condition, results in a bending stress, on the bolts, in an opposite direction. Consequently, each start up cycle of the steam turbine may result in a fatigue cycle of the cover and the bolts, first in one direction and then in the opposite direction.

Accordingly there is a need in the art for a pressure vessel cover that reduces stress on the cover and on the bolts that fasten the cover to the pressure vessel.

BRIEF DESCRIPTION OF THE INVENTION

Disclosed is an apparatus that relates to a pressure vessel cover comprising, a periphery of the cover, a first surface of the periphery for sealing on a first side of the cover, a second surface on a second side of the cover opposite from the first side, a third surface on the first side of the cover and located radially inwardly from the periphery, said third surface being displaced axially from the first surface in the direction of the second surface, a fourth surface on the second side of the cover radially inwardly positioned from the periphery; and a bore through the cover in fluidic communication with the third surface and the fourth surface.

Further disclosed herein is a method that relates to distributing stress in a bored pressure vessel cover, comprising, transmitting stress applied to an interior surface of the cover surrounding the bore symmetrically to a thickness of a periphery of the cover.

BRIEF DESCRIPTION OF THE DRAWINGS

The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike:

FIG. 1 depicts a partial cross sectional view of a valve assembly disclosed herein;

FIG. 2 depicts a perspective view of a cover disclosed herein; and

FIG. 3 depicts a cross sectional view of the cover of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a steam valve assembly in accordance with an embodiment of the invention is generally shown at 10. The valve assembly 10 includes a valve body 14 with an inlet 18 and an outlet 22. A seat 26, sealably attached to the body 14, may receive such as a control valve head 30 or a stop valve head 34. Flow through the valve 10 from the inlet 18 to the outlet 22 flows through the annular opening between the heads 30, 34 and the valve seat 26. Flow of steam is halted when either valve head 30, 34 contacts the seat 26. The stop valve head 34 is actuated through the stop valve stem 38 by a stop valve actuator (not shown). The stop valve is designed to be fully opened or fully closed and may be used as an emergency shut off valve. The control valve head 30 is actuated through control valve stem 42 by a control valve actuator (not shown). The control valve is designed to allow for variable levels of closure depending upon how far the control valve head 30 is from the seat 26. The forgoing construction is sometimes referred to as an in-line combination valve, since the control valve and the stop valve share a common axis.

The control valve stem 42 slidably engages with a bushing 46 pressed into a bore 50 through a cover 54. The control valve actuator is positioned relative to the cover 54 by an actuator yoke 58 that abuts a yoke surface 62, on the cover 54, thereby aligning the control valve stem 42 with the bushing 46 and the bore 50. The yoke surface 62 may be flat to further facilitate aligning the yoke 58 with the bushing 46 and the bore 50. The yoke surface 62 is substantially perpendicular to the bore 50 and bushing 46. The perpendicularity provides alignment to the actuator such that the stem 42 moves through the cover 54 in a perpendicular manner to the yoke surface 62. The control valve stem 42 is in operable communication with the head 30 such that movement of the stem 42 results in movement of the head 30, which directly controls the annular flow area between the head 30 and the valve seat 26. The annular flow area controls the flow rate of the steam through the valve 10. Thus control of the flow rate of the steam is directly controllable by controlling the control valve actuator.

The control valve may completely stop flow of the steam by extending the stem 42 and moving the control valve head 30 into contact with the seat 26. An accurate alignment of the head 30 with the seat 26 is necessary to assure valve closure as the head 30 contacts the seat 26. Such alignment within the pressure vessel is provided to the head through a balance chamber 66. The balance chamber 66 abuts a third surface, also referred to herein as chamber surface 70, which may be flat, on the interior of the cover 54 that is perpendicular to the bore 50. An exterior cylindrical wall 80 of the head 34 slidably engages an internal cylindrical wall 76 of the balance chamber 66, thereby guiding the head 30 in a direction perpendicular to the flat surface 70. Accordingly, by making the flat surfaces 70, 62 perpendicular to the bore 50, the actuator (not shown), the actuator yoke 58, the control valve stem 42, the balance chamber 66 and the head 30 are all kept in alignment relative to the cover 54. Consequently, the cover 54 is an important component in maintaining accurate alignment of the head 30 with the seat 26. The valve body 14 also provides alignment of the head 30 with the seat 26 since both the seat 26 and the cover 54 are attached directly to the body 14.

Referring to FIGS. 2 and 3, the cover 54 has a periphery 74 with a plurality of holes 78 that extend axially therethrough. The holes 78 receive bolts 82 that threadably engage with threaded holes 84 in the valve body 14, thereby attaching the cover 54 to the valve body 14. In addition to the structural attachment of the cover 54 to the body 14, a sealing of the cover 54 to the body 14 is also achieved. A first surface, referred to herein as sealing surface 86, which may be flat, on the cover 54, sealably engages with a seal surface 90 of the opening of the pressure vessel body 14. A gasket (not shown) may also be included between the surfaces 86 and 90 to facilitate sealing. The sealing surfaces 86 and 90 may be in the same plane as the holes 78 and 84 or may be stepped to a separate plane without deviating from embodiments of the present invention.

Maintaining a seal against the high pressure of the steam requires uniform distribution of the forces generated by the bolts 82 on the cover 54. A second surface, referred to herein as surface 94, which may be flat, of the cover 54, is substantially perpendicular to the holes 78 and provides a surface for the heads of the bolts 82 to seat against. The flat surface 94 is substantially parallel to the flat surface 86 but is on an opposite side of the periphery 74 from the seal flat surface 86. All four of the flat surfaces 62, 70, 86 and 94 are substantially parallel to one another thereby positioning the bore 50 perpendicular to the surface of the cover 54 as well as perpendicular to the seal surface 90 of the body 14 providing alignment of the head 30 with the seat 26 as described above.

In addition to providing alignment of the valve components, the cover also must withstand stresses that arise from the high temperatures and pressures, to which the internal surfaces of the cover are subjected. One embodiment of the invention distributes these stresses by doming the cover 54. Specifically, a central portion 100 of the cover 54, containing among other things the flat surfaces 62, 70 and bore 50, is displaced axially from the periphery 74. A generously curved internal surface 104 connects the displaced flat surface 70, of the central portion 100, with the flat surface 86, of the periphery 74. Similarly, a generously curved external surface 108 connects a flat surface 62, also referred to herein as a fourth surface, portion of the surface 108, of the central portion 100, with the flat surface 94, of the periphery 74. The thickness between the flat surfaces 86 and 94, the curved surfaces 104 and 108 and the flat surfaces 70 and 62 are substantially equal to one another. Stated another way, the cover 54 has a substantially constant wall thickness throughout.

The domed shape of the cover 54 reduces the stresses throughout the cover 54, as well as the stresses that act on the bolts 82 when compared to a cover that is flat and not dome shaped. During heat up, for example, when a cold assembly is exposed to hot steam, the internal surfaces 70 and 104, which are in direct contact with the steam, heat up first and thus expand first. Thermal bending, which may cause a dishing in a flat cover as described in the background above, does not cause dishing with the domed cover. Thermal bending does not occur in the domed cover 54 due to the geometry of the domed shape. Specifically, the expanding internal flat surface 70, for example, is positioned substantially midway between the flat surfaces 86 and 94 of the periphery 74, thereby distributing this expansive load symmetrically into the full thickness of the periphery 74, instead of into the surface 86 alone. By distributing the stress symmetrically there is little force acting to deform the cover 54 into a dished shape. Additionally by distributing the load into the center of the periphery 74 the full thickness of the periphery 74 is resisting the load resulting in a reduction in the overall stress of the periphery 74.

Loading applied to the bolts 82 from the cover 54 is also reduced by the domed shape of the cover 54. Since, as described above, the periphery 74 is under less total stress, the periphery 74 does not expand as much. Consequently, bending forces applied to the bolts 82 by the cover 54 are reduced since the holes 78 remain parallel to and in alignment with the holes 84, in the body 14.

The domed shape of the cover 54 also reduces the stresses generated by pressure acting on the cover 54. This is due to the increase in stiffness of the cover 54 that results from the domed shape, which is similar to the stiffening that results when ribbing is added to any flat shape.

The improved load distributing characteristics of the cover 54 permit the cover 54 to use a thinner overall thickness and less total material to fabricate than a flat cover for the same application. Additionally, since the flat surfaces that may need to be machined on the domed cover 54 include less total area than a comparable flat cover, for example, the costs for material, time and labor to perform such machining operations should also be less.

A feature referred to as “leakoff,” which allows for the porting off of high pressure/high temperature steam, is sometimes desirable during throttling of control valves. Therefore, the domed cover 54 may include a port 112 in fluidic communication with the bore 50 and an exterior circumference 116 of the periphery 74. Port 112 also may allow for leakoff of the steam to atmosphere or to a steam recovery unit through piping (not shown) connected to a flange 120 bolted to the circumference 116. In the domed cover 54, the port 112 may be formed at an angle relative to the flat surface 86 in order to avoid fluidically connecting the port 112 to any of the surfaces 86, 94, 62, 70, 194 and 108.

Some embodiments of the invention may include some of the following advantages: less cost to manufacture, less total mass, less time to fabricate, more durable, longer life, lower stresses internally and on fastening bolts, internal and external mounting surfaces for valve component and reduced dishing and bowing.

While the embodiments of the disclosed method and apparatus have been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the embodiments of the disclosed method and apparatus. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the embodiments of the disclosed method and apparatus without departing from the essential scope thereof. Therefore, it is intended that the embodiments of the disclosed method and apparatus not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out the embodiments of the disclosed method and apparatus, but that the embodiments of the disclosed method and apparatus will include all embodiments falling within the scope of the appended claims. 

1. A pressure vessel cover, comprising: a periphery of the cover; a first surface of the periphery for sealing on a first side of the cover; a second surface on a second side of the cover opposite from the first side; a third surface on the first side of the cover and located radially inwardly from the periphery, said third surface being displaced axially from the first surface in the direction of the second surface, a fourth surface on the second side of the cover radially inwardly positioned from the periphery; and a bore through the cover in fluidic communication with the third surface and the fourth surface.
 2. The pressure vessel cover of claim 1, further comprising: a port through the cover in fluidic communication with the bore and an outer circumference of the periphery.
 3. The pressure vessel cover of claim 1, further comprising: a plurality of through holes in the periphery in fluidic communication with the first surface and the second surface.
 4. The pressure vessel cover of claim 1, wherein: an axial distance from the first surface to the second surface is substantially equal to an axial distance from the third surface to the fourth surface.
 5. The pressure vessel cover of claim 3, wherein: the first surface is substantially flat.
 6. The pressure vessel cover of claim 5, wherein: the plurality of holes are substantially perpendicular to the first surface and the perpendicularity of the plurality of holes is unaltered by an application of heat and pressure to the third surface.
 7. The pressure vessel cover of claim 5, wherein: the third surface is substantially flat.
 8. The pressure vessel cover of claim 7, wherein: the fourth surface is substantially flat.
 9. The pressure vessel cover of claim 8, wherein: the first, third and fourth surfaces are substantially parallel to each other.
 10. A method of distributing stress in a bored pressure vessel cover, comprising: transmitting stress applied to an interior surface of the cover surrounding the bore symmetrically to a thickness of a periphery of the cover.
 11. The method of claim 10, further comprising: maintaining substantially a constant wall thickness throughout the cover.
 12. The method of claim 10, further comprising: maintaining perpendicularity of a plurality of mounting holes through the periphery with a sealing surface of the periphery throughout temperature and pressure induced stressing of the cover. 